Conventional photographic silver halide emulsions contain discrete silver halide microcrystals (commonly referred to as grains) in a dispersing medium. The grains are typically formed by reacting silver and halide ions in an aqueous medium. A common reaction is as follows: EQU AgNo.sub.3 +Mx.fwdarw.AgX+MnO.sub.3
where PA0 The reaction is referred to as a precipitation, since the silver halide is partitioned into a separate phase (the grains), but the grains remain dispersed in the aqueous medium. To avoid agglomeration of the grains, a peptizer (typically gelatin or a gelatin derivative) is incorporated in the dispersing medium. To eliminate the soluble by-products of precipitation (e.g., MNO.sub.3), it is common practice to coagulate the gelatino-peptizer, thereby phase separating the gelatino-peptizer containing the grains dispersed therein from the remainder of the aqueous solution. Typically, the coagulated emulsion is washed to remove soluble salts, and the emulsion (the peptizer and the grains) is then again dispersed in water. The peptizer prevents the grains from agglomerating during coagulation and washing. After washing, the photographic emulsion is typically sensitized and prepared for coating as a layer in a photographic element by the incorporation of various addenda (e.g., stabilizers and antifoggants) along with binder, which also typically includes gelatin or a gelatin derivative. The peptizer and binder are commonly collectively referred to as photographic vehicle. The photographic vehicle forms a continuous phase of the photographic emulsion layer, and the grains are discretely dispersed in the vehicle.
M is ammonium or an alkali metal and
X is a photographic halide (i.e., Cl, Br and/or I).
Occasionally grains are formed or grown in the presence of antifoggants, stabilizers or spectral sensitizing dye, as illustrated by Research Diclosure, Vol. 365, September 1994, Item 36544, I. Emulsion grains and their precipitation, D. Grain modifying conditions and adjustments, paragraph (6). Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
Notice that in conventional emulsion preparations grain agglomeration or clumping is not allowed to occur. The terms "agglomeration" and "clumping" are here employed to indicate bringing separate grains into direct contact one with the other. That is, there is no peptizer separating the grains. The term "coagulation" herein and most commonly refers to precipitating the grains and peptizer together from an aqueous medium. While the term definitions herein adopted are consistent with the terminology of the art in most instances, the fact is that the art has employed a variety of terms, additionally including terms, such as flocculation, sedimentation, and coalescence, often with different meanings. Therefore, the teachings of the art must be considered carefully based on the substance of teachings rather than the choice of one adjective or another.
Mignot U.S. Pat. No. 4,334,012 illustrates an approach to growing silver halide grains to larger sizes in the absence of peptizer while avoiding agglomeration of the grains.
It has been speculated that discrete grains may occasionally be produced by the coalescence of two or more discrete grains. As the term "coalescence" is here employed, the difference between grains formed by coalescence and agglomerated grains is that grains formed by coalescence appear to be unitary, discrete grains, whereas agglomerated grains are aggregations of grains. For example, in Maskasky U.S. Pat. Nos. 5,264,337 and 5,292,632, which disclose unitary tabular grains, one speculation is that the tabular grains may be the result of coalescence of grain nuclei during precipitation.
When a silver halide emulsion is imagewise exposed, the exposed grains are rendered developable or, in direct-positive emulsions, nondevelopable. Larger grains have larger projected areas and hence a better opportunity to capture photons during imagewise exposure than finer grains. Also, larger grains make larger contributions to image formation than finer grains. Larger grain sizes are recognized to impart higher levels of photographic sensitivity.
Beginning in the early 1980's and continuing to the present, there has been considerable interest in tabular grain emulsions. Kofron et al U.S. Pat. No. No. 4,439,520 is representative. Among the post-discovery rationalizations of tabular grain emulsion performance advantages has been the observation that tabular grains exhibit a high ratio of grain surface area to volume. The surface to volume ratio is increased as the aspect ratio, the ratio of equivalent circular diameter (ECD) to grain thickness, increases. Thus, tabular grains can range up to very large sizes, with mean ECD's of up to 10 .mu.m being accepted as the practical upper limit of photographic utility.
It is, of course, not just the high surface to volume ratio of tabular grains that render them attractive. Surface to volume ratios equal to and higher than those of tabular grains are readily provided by fine grain emulsions. Unfortunately, the limited photographic speeds of fine grain emulsions have precluded their substitution for larger grain emulsions.